1. Field of the Invention
The present invention relates to biodegradable devices and methods for fabricating said devices. More particularly, the present invention relates to a biodegradable medical device with enhanced mechanical properties and/or pharmacological functions.
2. Discussion of the Related Art
There exists a large body of literature for the extensive uses of biodegradable and biocompatible polymers for pharmaceutical and medical device applications. Biodegradable polymers are finding increasing uses in medical devices. The safety for use in humans of these biomaterials is evident in the forms of bioabsorbable sutures, controlled release dosage forms such as Lupron depot, etc. Biodegradable materials such as Poly L-Lactic Acid (“PLLA”), Poly D,L-Lactides (“PDLA”), Poly Lactic Glycolic Acid (“PLGA”), Polycaprolactone (“PCL”), Poly Lactide-co-caprolactone (“PLA/CL”), or Poly Lactide/glycolide-co-dioxanone (“PLGA/DO”) are known to degrade under physiological conditions Current generation bioabsorbable polymers such as Poly Lactic Acid (“PLA”), Poly(glycolic)acid (“PGA”), which are aliphatic polyesters of poly(α-hydroxy acids), and Poly Lactic Glycolic Acid/Poly(Lactide-co-glycolide) (“PLGA”), as well as copolymers of PLA, PGA, or PLGA with caprolactone or dioxanone have been used as materials in medical devices with the goal of making the device resorbable and/or absorbable. Specifically these polymers and the related copolymers are the most common bioabsorbable polymers and have been used for the matrices and/or drug carriers for drug eluting stents.
Some examples of such use cited in both patents and publications include U.S. Pat. No. 5,977,204 (incorporated by reference) wherein a biodegradable implant material that comprises a bioactive ceramic is disclosed. The '204 patent discloses blends of surface-passivated bioceramic and biodegradable polymers which include mainly large particles of large porosity for dental and orthopedic applications. U.S. Pat. No. 6,244,871 (incorporated by reference) is another item of some interest in that it discusses Bioactive Glass compositions and methods of treatment using bioactive glass as well as a combination of bioglass and drug delivery vehicles for dental applications. U.S. Pat. No. 6,197,342 (incorporated by reference) discloses the use of biologically active glass as a drug delivery system as well as disclosing a method for impregnating bioglass with drugs for bone applications. U.S. Pat. No. 6,086,374 (incorporated by reference) discloses a Method of treatment using Bioactive glass wherein the treatment of tooth decay is sought to be addressed by using a combination of bioglass and drugs.
In publications such as Wilson, J, et. al., entitled Toxicology and biocompatibility of bioactive glass, JBMR, 1981, 15: 805, and/or Hench, L, et. al., entitled Biocompatibility of orthopedic implants, vol. 2, Boca Raton, Fla. CRC Press, 1982, P 67-85, and/or Greish, Y. and Brown, P. entitled Characterization of bioactive glass-reinforced HAP-polymer composites, Three dimensional, bioactive, biodegradable, polymer-bioactive glass composite scaffolds with improved mechanical properties that support collagen synthesis and mineralization of human osteoblast-like cells in vitro are discussed.
More recently Poly Lactic Glycolic Acid (“PLGA”) and other materials have been proposed as materials for stent and drug eluting stent applications. In parallel, bioactive glass and bioceramics are also used for medical device applications in areas of bone replacement and dental care. It is also known that Bioceramics such as BioGlass are commonly used in dental and bone replacement applications and have excellent biocompatibility and safety history with the Food & Drug Administration (“FDA”) and that regulatory filings on such products with the FDA exist. While Bioactive glasses have advantages such as bonding rapidly with bone and soft tissues, the disadvantages of bioactive glasses are their brittleness, which limits their uses in weight bearing applications.
Typically these polymeric materials have a very high degree of elasticity and tend to recoil after crimping or expansion. Having a low recoil property is one of many important factors in stent design, thus the high recoil of polymeric materials may not be advantageous. Normally PLLA and PLGA copolymers with a high percentage of LA (Lactic Acid) content as compared to GA (Glycolic Acid), for example a 95%:5% ratio of LA to GA respectively, which results in the copolymer being very brittle and may not easily allow processing into the desired shapes for medical device applications. Although PLGA is more elastic, the mechanical properties such as tensile strength, storage and Young's moduli are decreased with increasing amount of GA or CL (caprolactone). The mechanical properties of Biodegradable polymeric materials may also be negatively impacted by the presence of moisture; in particular, moisture may tend to reduce the modulus of the material. Furthermore, both PGA and PLA copolymers all release acidic products upon degradation resulting in localized acidic conditions in the area of the degrading implant.
Further, there is a need to address and improve filtering devices such as Distal protection devices or Vena Cava filters, both of which whose primary function is to capture and prevent embolic debris from closing off a vessel and ultimately causing tissue death and potentially a heart attack or stroke. With filtration devices such as distal protection and vena cava filters, as the mesh and or pore size of the filtering aspect decreases, more embolic material may become trapped in the filtering mechanism, thereby increasing the load on the filtering portion. While small emboli (typically smaller than 100 microns) are not a major concern because of the body's natural ability to enzymatically degrade, digest or lyse the emboli, the embolic load on the filter itself can be overloaded and result in formation of a thrombus if the blood flow is significantly slowed to the point which allows for a thrombus formation. This thrombus formation if allowed to go unchecked would eventually close off flow to all downstream tissue, which relies upon the vessel in question.
Some other common difficulties with biodegradable stents that may exist are the ability to control the degradation rate of these materials.